1. Field of the Invention
The field of the invention is that of optical devices comprising an integrated semi-conductor laser emission source and an integrated optical isolator. These devices are used mainly in the field of high-speed digital telecommunications.
2. Description of the Prior Art
It is known that a laser source can be rendered unstable by a stray optical beam emanating from the initial emission beam and reflected by an optical surface external to the cavity of the laser. Subsequently in the text, a light emission source comprising an optical amplifier of laser type will either be called a laser source or laser. To decrease this phenomenon, an optical isolator is disposed at the exit of the laser. Its function is to allow through the light emanating from the laser and to eliminate any stray light coming in the opposite direction.
Basically, an optical isolator implements a non-reciprocal optical effect making it possible to ensure this function. The best known is the Faraday effect. Subjected to an external magnetic field, certain so-called magneto-optical materials rotate the plane of polarization of the light in a different direction according to the direction of propagation of the light. Thus, by placing this type of material between appropriately disposed polarizers, it is possible to transmit the light propagating in a first direction and to block it in the opposite direction. These devices are rather unsuitable for optical devices comprising semi-conductor lasers insofar as they require a certain number of components that are difficult to integrate into devices whose dimensions are of the order of a millimeter.
Finally, it is possible to use optical isolators with non-reciprocal absorption. It is known that the optical index is complex. It comprises a real part and an imaginary part which is proportional to the absorption of the material. But, in the presence of a magnetic field, the optical index of certain materials of ferromagnetic type depends on the direction of propagation of the light. FIG. 1 presents the variations of the real and imaginary parts of the optical index of the optical mode termed transverse magnetic TM of this type of material as a function of the direction of propagation of the light. In the absence of a magnetic field, the optical index in the material 10 equals n. In the presence of a magnetic field symbolized by a black arrow, according to the direction of propagation of the light symbolized by a straight barred arrow, the index n becomes n↑ in a first direction of propagation and n↓ in the opposite direction. The difference between the two imaginary parts I.P. of the indices n↑ and n↓ gives the isolation ratio I.R. Consequently, according to its direction of propagation, a light beam will be more or less absorbed by this type of material.
The latter isolators are very suitable for integration with optical devices comprising a semi-conductor laser source. In this case, they are generally integrated with an amplifying structure of SOA type, the acronym signifying Semiconductor Optical Amplifier. Thus, the optical amplification provided by the SOA compensates, in a first direction of propagation, for the weaker absorption of the material. In the opposite direction of propagation, the absorption remains predominant and attenuates the light beam so as to avoid returns.
FIGS. 2, 3 and 4 represent a device of this type. FIG. 2 is a longitudinal sectional view and FIGS. 3 and 4 represent transverse sectional views. The structure essentially comprises two parts which are on the one hand a semi-conductor laser 10 and on the other hand an absorption optical isolator 20, the whole assembly lying on a common substrate 1.
The semi-conductor laser 10 is a so-called buried stripe structure also called a BRS structure, the acronym standing for Buried Ridge Stripe. The sectional diagram of such a structure is represented in FIG. 3. It comprises essentially:                The common substrate 1 made of n-doped semi-conductor material. This first substrate is generally made of InP;        An active part 2 formed by a stripe of rectangular cross section, the lower face of this active part lying on the first substrate 1. The active part has an optical index greater than that of the layers which surround it. It is of small section, of the order of a micron or of a few microns, and is generally made of GaInAsP or of GaInAlAs or of GaInNAs;        A layer 3 made of p-doped semi-conductor material. This layer is also made of InP and it completely covers the lateral faces and the upper face of the active part 2. Its thickness is a few microns, typically 2 to 3;        An electrical contact layer not represented in the figures disposed under the first substrate 1 and an upper electrical contact layer 11 disposed on the second substrate 3. These layers are generally made of InP/InGaAs. The electrodes are disposed on these contact layers. They convey the current necessary for the operation of the laser. Generally, the electrodes are made of gold platinum alloy.        An implantation of protons is generally carried out in the p-doped layer, on either side of the active zone, to improve the electrical confinement. This implantation is represented by + symbols in the various figures.        
This configuration makes it possible to ensure, at one and the same time:                Confinement of the carriers injected into the stripe if the difference in forbidden bandwidth between the material of the first substrate and that of the second substrate is sufficient;        Bidirectional guidance of the light if the difference in optical index between the material of the first substrate and that of the second substrate is also sufficient.        
These lasers generally emit in the near infra-red at wavelengths neighbouring 1.3 microns or 1.5 microns. It is possible to append additional layers so as to carry out other functions. In particular, by adding an optical grating on the active layer, it is possible to produce a so-called distributed feedback DFB laser.
The optical isolator 20 is also a so-called buried stripe structure. The sectional diagram of such a structure is represented in FIG. 4. It comprises essentially and successively:                The substrate 1 made of semi-conductor material made of n-doped InP common with that of the laser;        An active part 2 also common with that of the laser;        A layer 3 made of p-doped semi-conductor material. This layer also common with that of the laser is however, in the isolator part, of much smaller thickness so that the magnetic field is closest to the active layer. Its thickness does not exceed a few tenths of a micron.        An electrical contact layer not represented in the figures disposed under the first substrate 1 and an upper electrical contact layer 21 disposed on the second substrate 3. The electrodes are disposed on these contact layers. The electrodes convey the current necessary for the amplification of the laser radiation.        A ferromagnetic material layer 4 which can, for example, be made of Iron-Cobalt alloy. The magnetization of the material is symbolized by an arrow in FIG. 4 and by circles comprising a central cross in FIG. 2. This layer ensures at one and the same time magnetization of the active layer and electrical contact.        
The implantation of protons in the layers 1 is symbolized by the symbol +.
The optical beam emitted by the semi-conductor laser propagates through the active layer common to the laser and to the isolator and the electrical contact layer. It is represented by straight barred arrows in FIG. 2 and by a series of faint-line concentric ellipses in FIGS. 3 and 4.
This configuration makes it possible to ensure at one and the same time the amplification of the light beam emanating from the semi-conductor laser and the absorption of the stray light coming in the opposite direction to the direction of propagation as indicated by the straight barred arrows of FIG. 2. The lengths of the arrows are representative of the amplitudes of the optical beams. With this type of configuration, it is possible to ensure an isolation ratio of greater than 25 dB with an optical isolator length of the order of 1 to 2 millimeters.
However, this type of configuration exhibits several major drawbacks. The electrical contact layer being very near to the active zone, it disturbs the propagation of the optical mode and causes significant optical losses. The structure of the electrical contacts not being optimized, significant electrical losses can occur, causing local heating. Finally, the small thickness of the substrate 3 necessary for the operation of the isolator poses technological production problems that are tricky to control.
To solve these difficulties it is of course possible to change the arrangement of the structure. In this case, the possibility of integrating into one single component at one and the same time the BRS laser and the optical isolator is lost. Moreover, this type of structure has lower performance.